A number of devices are known in the prior art which allow for drainage of a liquid from a surface while at the same time forming a liquid barrier to prevent the release or escape of gases from the sewer into the ambient atmosphere. Examples of prior art disclosing such feature are U.S. Pat. No. 4,682,907, issued Jul. 28, 1987 and U.S. Pat. No. 4,776,722, issued Oct. 11, 1988. These patents both relate to self-sealing sewer cover assemblies for preventing flammable gases from being discharged from a sewer and for preventing the ignition of gases in a sewer by flames and sparks in the area adjacent to the sewer inlet. The apparatus disclosed in these patents includes a cover assembly which may replace or be used in combination with an existing cover or grating on a sewer inlet and may be inserted into the sewer inlet. The apparatus construction disclosed in these patents provide a liquid barrier between the gases contained in the sewer and the ambient air adjacent to the sewer inlet which prevents the flow of gases from the sewer to the ambient air surrounding the sewer inlet.
U.S. Pat. No. 3,516,541, issued Jun. 23, 1970, discloses a prefabricated drain safety device which is removably mounted in a vertical drain structure. The device, when partially filled with water, provides a seal preventing upward discharge of explosive vapors through the drain structure into the ambient atmosphere.
U.S. Pat. No. 3,556,993, issued Jan. 19, 1971, discloses a surface drain with water trap. The device consists of a flush surface drain with a removable grid. Within the outer casing of the drain a sump is formed. A removable baffle is spaced from an inner wall of the sump so as to leave a gap between the baffle and sump wall and form a trap within the sump.